1. Field of the Invention
The invention is concerned with magnetic devices. A category of particular significance, known as bubble devices, involves nucleation and propagation of cylindrical single wall domains of polarization opposite to that of surrounding regions.
2. History
The trend to increasing miniaturization is common to development in the various circuit technologies. Advanced development is concerned with large-scale integrated silicon circuitry in the semiconductor technology, with light guides and integrated optics in optical communications and with bubble devices in magnetic technology.
Bubble technology had its beginnings over a decade ago and has advanced through successive levels of sophistication. State of the art commercial devices, based on 2-4 .mu.m diameter bubbles and 100K cells of 16 .mu.m dimension, have been in use for some time.
An early quest for bubble-supporting material of appropriate magnetic properties has, at this time, resulted in near universal acceptance of thin layers of magnetic garnet supported by nonmagnetic garnet substrate. Such layers are almost invariably produced by liquid phase epitaxial (LPE) growth under deliberately super-cooled conditions as disclosed, e.g., in U.S. Pat. No. 3,790,405, issued Feb. 5, 1974 to H. J. Levinstein. Commercial devices are based on LPE layers of garnet in which desired anisotropy is tailored by multiple occupancy of dodecahedral sites and in which magnetic moment, 4 .pi.M, is tailored by appropriate dilution of iron in tetrahedral sites. Growth is on gadolinium-gallium-garnet (GGG) substrates. Necessary magnetic properties have been available for field access devices (arrays in which bubble propagation is responsive to rotating magnetic fields coupled with a variety of patterned permalloy overlays) as disclosed, e.g., by A. H. Bobeck et al., "Magnetic Bubbles--An Emerging New Memory Technology", Proceedings of the IEEE, Vol. 63, No. 8, August 1975, pp. 1176-1195. Such layered material has also met requirements for new generations of current access devices at present experimental design rules.
While virgin LPE layers have generally sufficed as indicated, desired properties have sometimes been enhanced by subsequent treatment. Treatment has included annealing, diffusion, ion implantation, etc. Ion implantation has played a special role in bubble device fabrication as disclosed, e.g., in U.S. Pat. No. 3,792,452, issued Feb. 12, 1974 to M. Dixon et al., and by W. A. Johnson et al., "Differential Etching of Ion-Implanted Garnet", J. Appl. Phys., Vol. 44, No. 10, October 1973, pp. 4753-4757. The book by G. Dearnaley et al., Ion Implantation, North-Holland Publishing Company, 1973 may serve as a general background reference.
Extremely sensitive site occupancy and/or strain balance responsible for necessary anisotropy may be altered by subtle changes produced by ion implantation. From an experimental standpoint, a variety of implantation boundaries have served to modify surface or embedded regions. Implantation has served to define preferred propagation routes, as well as to process entire layers. One aspect of ion implantation is well known. This involves the production of a capping, surface-damaged region produced by bombardment by low energy heavy ions. In general, such devices are provided with a surface region of in-plane easy magnetization direction designed to avoid "hard" bubbles. (Hard bubbles evidence a canted response to an applied field and tend to stray from prescribed routes.)
LPE garnet layers produced on dipped substrate wafers are characterized by thickness uniformity of a fraction of a .mu.m as well as excellent composition uniformity. As in the past, state-of-the-art processing will be inadequate for new generation devices. Miniaturization requiring bubble diameter decrease to below 2 .mu.m and eventually to submicron, is attended by more stringent uniformity requirements. Functional magnetic layers are needed which are of the same approximate thickness as bubble diameter; layers this thin must be even more closely controlled, and compositional inhomogeneity which formerly could be ignored must now be avoided.
Compositional inhomogeneity of initially grown material almost invariably results from distribution coefficients, k.noteq.1. (Composition of growing solid differs from melt composition, thereby resulting in depletion or enrichment of interfacial melt during initial growth.) Steady state growth results in a stable, diffusion-limited .delta.-layer in the liquid, providing for gradual change of composition from solid layer to bulk melt composition. The thickness of the material grown under nonsteady state conditions may be of the order of tenths of a .mu.m. As the total layer thickness decreases, material grown under nonsteady state conditions (which is characteristic of liquid growth techniques, such as, e.g., LPE) becomes determinative of critical magnetic properties--such as, e.g., bubble diameter, anisotropy, and other parameters.